The present invention is in the field of helicopters, and more specifically relates to a manufacturing process and fixture for use in fabricating the main rotor blade of a helicopter.
Contemporary main rotor blades are hollow, being formed of a reinforced leading edge which is attached to a skin which defines the aerodynamic surfaces. From a structural standpoint to prevent erosion, it is desirable that the leading edge be made of steel, while the skin covering the remainder of the blade be made of aluminum.
There are advantages to bonding, rather than riveting, the components of the rotor blade together to form a unitary structure, but the bonded blades of dissimilar metals have not proven satisfactory. When rotor blades are bonded of dissimilar metals having different coefficients of thermal expansion, using conventional bonding techniques, the blades are usually found to be warped or bowed when they are removed from the bonding fixture.
This adverse result is caused by the fact that the bonding takes place while the blade is at an elevated temperature required to cure the adhesive so when the blade is cooled back down to room temperature, the material having the higher coefficient of thermal expansion will shrink the most. For example, if the aft portion of the blade is made of aluminum and the forward portion of steel, the blade will bow aft when it is removed from the bonding jig, assuming the jig is rigid and straight. In addition to bowing of the blade, which affects its balance and control of feedback, the blade will also have detrimental internal stresses bonded into its structure which can lead to premature fatigue failure. Thus, the use of techniques known to the prior art does not result in the most desirable rotor blade when bonded of dissimilar metals.